The present invention relates to a color correction method and apparatus for faithfully reproducing the original color image in a color reproduction system, and more particularly, to a color correction method and apparatus in which a color space is adaptively divided and color compensation coefficients are produced with respect to the division of the color space.
Recently, a significant difference in color reproduction among systems of using various media has come to light. If we compare an original image which may be picked up by a scanner or shown through a monitor with a reproduction image which may be obtained by printing, a considerable difference between the original and reproduced images emerges. This is primarily due to a difference in the respective color reproduction methods, such that monitors and scanners typically adopt an additive color system while printers typically use a subtractive color system. In addition, a difference between the original image and the reproduction image can originate from the different color gamuts of the color reproduction apparatuses. For reducing this color difference, a color transformation according to the input color gamut is performed for printing.
Generally, in a color correction method, the Beer-Lambert and Neugebauer equations have been proposed in order to estimate color reproduction by using analytic modeling. These equations, however, have a disadvantage in that they cannot truly compensate a non-linearity of the apparatus used which occurs in practice.
On the other hand, an empirical method, whereby a color reproduction process is accomplished within a "black box" and a correlation between the input and output thereof is observed, is widely used in color correction. As such, there are a method using a multiple regression analysis and a method using a look-up table. Though the former method, in which input and output data are processed statistically to be represented by an optimal input/output relationship, is widely used in recent times, it has a drawback in that it is not applicable to all color gamuts. In addition, the latter method can perform a reliable color correction for gamut colors due to the usage of the look-up table data sampled in every gamut color, but it has a disadvantage in that it requires numerous measuring colors and a detailed interpolation method for its accuracy and must have ample memory for storing the complete look-up table.
On the other hand, in the conventional color conversion method, a color transform matrix in the form of 3.times.3, 3.times.6 and 3.times.9 is obtained from the relationship between RGB input and RGB output using a multiple regression analysis. The original image is multiplied by the obtained color transform matrix. However, because the relationship between the input RGB space and the output RGB space is nonlinear and non-uniform, a color correction using only one color transform matrix leads to a deficient result. For solving this problem, Y. Miyake et. al have disclosed a paper entitled "Facial Pattern Detection and Color Correction from Television Picture for Newspaper Printing" (Image Science and Technology, 1990). In this paper, when a television image is reproduced by a newspaper printing system, the correction is made such that an input RGB space is divided into flesh tones and other colors in order to implement realistic flesh color, and a color transform matrix is obtained from the relationship between the RGB input and output for each color category using a multiple regression analysis.
Also, in another paper by Y. Miyake et. al ("A New Method for Color Correction in Hardcopy from CRT Images," Image Science and Technology, Vol. 37-1, pp 30-36, 1993), color correction is made such that an input color space is divided into five categories: flesh tones, gray, red, green and blue. In other words, the RGB coordinates are first projected onto a three-dimension plane based on the following equations (1) ##EQU1##
In addition, provided that p and q represent the coordinates of this plane for identifying a color position, p and q can be obtained from the coordinates according to the following equation (2) ##EQU2##
This conventional color correction method, in which flesh tones, gray, red, green and blue are equally divided and processed as described above, is difficult for showing accurately the output characteristics of a printer because the division is not suitable for the RGB space characteristics. This leads to a problem in that numerous errors are included in the color transform matrix itself, as obtained from the respective gamut levels.